1. Field of the Invention
This invention relates in general to a method and apparatus for optimizing the seek/settle performance of a storage device, and more particularly to a method and apparatus that measures the seek/settle performance for each individual head and selects the heads with the best seek/settle performance.
2. Description of Related Art
Disk drive units incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Transducer heads driven in a path toward and away from the drive axis write data to the disks and read data from the disks. The data is recorded in concentric, radially spaced data information tracks arrayed on the surfaces on the disks.
Servo position control is used to position these data heads in registration with the data information tracks. One type of servo system uses a dedicated transducer head to read position signals recorded in servo information on a disk surface. The data heads are arranged with the servo head for simultaneous movement relative to the data information tracks and the servo information tracks. To access the disk drive unit, a feedback controlled drive system locates the servo head in a desired position, thereby to locate a data head in registration with a specific data information track where data is to be written or read.
However, other systems have servo positioning information prerecorded on the disk in servo sectors angularly spaced around the disk and located in the data tracks. The servo information is read and processed by a digital control system to control the amount of current sent to the voice coil motor (VCM). In this manner the head is maintained on track during read and write operations and accurately moved across the tracks to read and write on all the tracks.
There is usually significant variation in the access performance between recording heads in the same disk drive. This variation has been observed since the beginning of embedded servo control in disk drives. History has shown in disk drives using ferrite heads that wider heads would normally have better seek and settle performance due to more precise position error signal (PES) registration. For example, U.S. Pat. No. 5,245,478, issued Sep. 14, 1993, to Earl Cunningham, incorporated herein by reference, uses a dedicated servo head that is three times wider than the data heads thereby yielding a much better servo performance.
According to Cunningham, position error signals provided by conventional arrangements are subject to problems leading to difficulties in accurate and consistent head positioning during disk access. The signal detected by a servo transducer head of conventional width is effected strongly by the abrupt phase jumps between adjacent tracks. As a result, a conventional head is incapable of providing a smooth, linear position error signal. In addition, the signal is low in amplitude and is undesirably sensitive to variations in parameters including written track density, servo head dimensional tolerances and misregistration between servo and data tracks. Another disadvantage of prior conventional designs is that the position error signal is adversely affected by head jitter in the circumferential or time direction as well as jitter in the radial direction that may accompany non regular bearing runout during writing of the servo tracks.
In addition to these problems, yet another difficulty with the standard servo head arises from the effect of the edge fringe areas separating the written servo information. The nonmagnetized regions between tracks acts as dead zones in which non-signal change is detected in response to increments of radial servo head movement. The severity of this undesired effect depends upon servo head width compared to track width. If the head width is equal to an integer number of track widths, no gain in position error signal is present when both edges of the head are aligned with edge fringe areas. For other widths, gain is seriously accentuated when either edge of the head is aligned with an edge fringe area. Therefore, Cunningham suggests using a servo head three times wider than data heads to overcome these disadvantages. However, today drive manufacturers are constantly challenging the track density and the heads are getting narrower and narrower. Thus, wide heads as taught by Cunningham for reading servo signals would undermine the task of increasing track densities.
Still, other observations show that some MR heads with instability and/or asymmetric read sensitivity profiles have degraded servo. In fact there are several factors that contribute to poor seek and settle performance, which again points to degradation of signal-to-noise-ratio (SNR) in the position error signal (PES). Degradation in PES SNR will cause the seek performance and especially the settle-performance to be impaired. Tracking performance will always suffer from poor SNR.
There are several factors affecting seek/settle performance. For example, MR head instability and irregular read sensitivity profiles may affect seek/settle performance. MR head instability and irregular read sensitivity cause changes in the PES translation and therefore decrease the PES SNR. In addition, seek/settle performance may be affected by individual disk surface-coercivity and variation in coercivity across the data band. Variation in coercivity will cause variation in the erase-stripes surrounding the servo patterns on a particular disk surface. This results in decreased SNR in PES.
A third cause of seek/settle performance degradation involves the dynamic conditions of individual head/suspension/arm caused by variations in components and manufacturing tolerances. These dynamic conditions may, for example, have highly underdampened resonances due to a poor swage joint or a tilted head. Thus poor head/suspension/arm dynamics will affect the seek and especially the settle performance.
A large protruding defect on one disk surface may also cause problems if hit by the slider or MR element. Thermal asperities (TA) and slider roll during seek are just two examples. This will affect the SNR of the PES in local defect areas.
Furthermore, there might be more turbulent flow on the two outside surfaces of the disk pack causing slider buffeting. Inside the disk pack, observations show that the air flow is much more laminar. Slider buffeting and roll will change flyheight and consequently the PES.
Finally, the flyheight variation of the individual head as it passes over the data area during a seek may affect seek/settle performance. Variation in flyheight will cause some variation in PES.
As can be seen from these factors, PES SNR may be easily affected. There are multiple scenarios one can contrive to argue why the seek and settle performance will vary as a function of which head is used during the access. The position error signal will be an ambiguous function of these scenarios. The variations between heads are based in tolerances of components, manufacturing, and environment. In a modern disk drive, the PES signal and available gray code are used to guide (closed-loop) the seeking data head on a radial velocity profile setup between starting track and the target track. The position error signal (PES) figures heavily in the estimation of the radial head velocity, and the ability of the servo system to follow the prescribed velocity profile with a minimal amount of error. The heads inability to closely follow the velocity profile will result in longer settling times. This causes poorer access performance.
It can be seen then that there is a need for a method that optimized seek/settle performance.